A Passive Optical Network (PON) is a communication system where data is transmitted bi-directionally over a fiber infrastructure, the Optical Distribution Network (ODN) in a point-to-multi-point configuration. It consists of an Optical Line Termination (OLT), which resides in a Central Office (CO). The OLT services a number of Optical Network Units (ONU) or Optical Network Terminations (ONT) residing at (or close to the) premises of the end users, typically connected in a star arrangement using optical power splitters. Since the physical medium is shared, the ONU's are scheduled by the OLT to transmit in the upstream direction in a Time Division Multiple Access (TDMA) manner.
PON systems allow for flexibility in the geographical area coverage of the ODN. This leads to large differences in the attenuation of the optical signal, when arriving at the receiver in the OLT, for different ONU's. The OLT receiver is therefore required to handle a large dynamic range of optical input powers—typically 10-25 dB.
Transmissions from different ONU's are made in fairly short bursts which make it important at the OLT side to adapt to input power variations quickly.
The geographical distance difference of different ONUs also means that there will be a difference in phase of the burst signals arriving at the OLT. After detection in the optical receiver at the OLT, the signal needs to be phase aligned to the local system clock at the OLT. This is done by a Clock- and Data Recovery (CDR) device.
The goal of this process is to find an ideal threshold for detection and an optimal point in time to sample each data bit. The device or devices involved in this process is usually referred to as burst-mode receiver (BMR), but other names exist.
FIG. 2 outlines the overhead fields in the burst transmission that aids the burst-mode receiver to achieve an optimal threshold and sampling point;
Guard time: This is a time slot when the “previous burst” transmitter turns off its laser and the “current burst” transmitter turns off its laser    Preamble: This is a training pattern that the burst-mode receiver uses to decide optimal threshold and sampling point    Delimiter: This field is used for achieving byte or word synchronization in the receiver.    The length of the overhead needs to be a trade-off between performance and protocol efficiency. Too long overhead leads to a penalty in protocol efficiency and less capacity is available to transfer real payload data.
To make the optical receiver able to quickly adapt to optical input powers varying as much as 10-25 dB, some sort of adaptive circuitry is necessary. The adaptive circuitry is sometimes termed “automatic gain control” or “adaptive threshold detection” but other variants exist. The adaptation process is normally helped by letting the ONU send out a training pattern, usually referred to as preamble, before the actual data transmission begins. The receiver can then adjust itself to be prepared for the actual data reception. The adaptive circuitry has an associated time constant which reflects the time it takes for the receiver to respond to a change in input amplitude. Normally, this time constant is longer when the difference in input amplitude, or dynamic range, is larger.
The burst-mode CDR process also associated with a time constant. This time constant depend heavily on the implementation. Normally, the burst-mode CDR process uses the preamble to decide where the optimal sampling point is.
The time it takes for both the optical receiver to adjust the threshold to the incoming burst signal and the time it takes for the burst-mode CDR to find the optimal sampling point to meet a specific target bit error ratio could be referred to as the optimal burst overhead time. The optimal burst overhead time for a given burst transmission, for a given optical receiver and for a given burst-mode CDR architecture is mainly decided by the parameters of the preceding burst.
Along the path from the transmitter at the ONU side to the receiver at the OLT side, one or several amplifiers (optical or O/E/O) may be placed to increase the total link budget and thus increase the reach of the system. This is often referred to as “long reach” PON or extended reach PON, but other names exist. These amplifiers may show transient behavior at the start of a burst transmission. Part of the preamble could be used to let these transient behaviors fade out.
For some existing burst-mode receiver implementations, the process of adapting to a new burst is helped by letting a device that knows about the timing of the upstream transmission provide a reset signal to the burst-mode receiver. The use of such reset signal may help to achieve fast adaptation times. However, it is not always the case that there is a device that easily can provide an accurate reset signal to the burst-mode receiver. One example of such case is a long-reach PON architecture where the burst-mode receiver is located outside the OLT main cabinet. Even though such a reset signal might be derived, at a certain cost of complexity, the adaptation time may still be a function of the difference in input amplitude between different bursts. In that case, it might still be beneficial to find an optimal overhead length. Also, when optical amplifiers are used, they may not be able to take advantage of a reset signal.
An existing solution to the problem of finding an optimal burst overhead is to adjust the preamble time based on the expected difference in received power from different ONUS, based on power measurements, as described in EP1793514. One drawback with this solution is that it requires signal strength measurement capabilities of the optical receiver, which requires external control signals to the receiver which adds to its complexity. in another solution, as described in EP1791275, the receiver is adjusted to the expected power level prior to the reception of the burst. This also requires external control signals to the receiver which adds to its complexity. A common problem with both these solutions is that, in a scenario when certain types of reach extenders is employed to increase the reach of the PON system (e.g. mid-span optical amplifiers), signal strength measurements may be difficult or costly to implement.
Another problem with these solutions is that they only focus on received signal power and does not take other link parameters into account; for example, transient signal degradation caused by a mid-span optical amplifier at the start of a burst.